Electrical conductor having composite central dielectric member



June 25, 1957 A. M. CLOGSTON ELECTRICAL CONDUCTOR HAVING COMPOSITECENTRAL DIELECTRIC MEMBER Original Filed March 7, 1951 FIG. 4

DIELECTRIC CONSTANT (E) or lNSULAT/NG uEo/uu BETWEEN INNER AND 011m?CONDUCTORS naa zo I r F INVENTOR .A. M. CL OGSTON SOURCE OR UT/L/ZAT/CWDEV/CE s. 5: -47 ATTORNEY fnited States Patent fitice 2,797,394.Eatented June 25, 1957 ELECTRICAL CONDUCTOR: HAVING COMPOSITECENTRAL'DIELECTRIC MEMBER Albert'M. Clogston, Morris Plains, NJL,assignor to 'Bell Telephone Laboratories, Incorporated, New "York, N.Y., a corporation of NewFYork Original applicationMarch 7, 1951, SerialNo.''21'4,393, now Patent No. 2,769,148, *dated October '30, 1956.Divided and this application April 1, I952,- SerinlN0. 27a,s09

2 Claims. (Cl.-333-96) 214,393, filed March 7, 1951, which issued asU.'S. Patent 2,769,148 on October 30, 1956.

It is an object of this invention to improve the current distribution inconductors of the above-meritioned-type and particularly to effect suchimprovement by the novel construction of the intermediatedielectricmember.

Due to the phenomenon known as skin effect, at'high frequencies thecurrent distribution through a conductor is not uniform. Consider,forexample, the case-of a two-conductor coaxialline to -which areapplied wavesof increasing frequency. Atzero and sufiiciently 'lowfrequencies the currents in the conductors-are substantially uniformlydistributed throughout andthe resistanceof the conductors and hence theconductor loss in the line'is at a minimum. With increasing frequency,the currentdistribution changes so that the current'density'is a maximumat the inner surface of the outer conductor and-at the outer surface ofthe inner conductor and decreases into the materialat a rate dependingon the frequency and the material. In the example given,"thecurrent-density may be negligible at the other surfaceof each conductor.From another point of view, the electromagnetic field between the twoconductors (where-theusefuhpower is transmitted) penetrates into theconductors with a field intensity decreasing with distance. *Thusthecurrent density (or field) in each conductor. is associated with a'powerloss that is a function of the distribution'of I current density (orfield) across the thickness ofithe conductor.

In the copending parent applicationidentified above, there are discloseda number of composite conductors, each of which comprises a multiplicityof insulated conducting elements of such number, dimensions,':anddisposition relative to each other and to the orientation ofthe-electromagnetic wave being propagated therein as to achieve a morefavorable distribution of -current=and field within the conductingvmaterial. In these composite conductors the power loss associated with.skin eiiect'in electrical conductors is greatly reduced.

Applying the principles of the invention of the parent application tothe case of the two-conductor coaxial line, one or both of theconductorsis formed of a multiplicity of thin metal laminationsv insulated from'one 'another by layers of insulating material, the smallest dimensionof the laminations beingin the direction perpendicular-"to both thedirection of wave propagation and'themagne'tic vector. Each metallamination is many'times (for example 10, 100 or even 1000 times)smaller thanthe factor 6 which is called one skin thickness or one-sk-in depth. The distance 6 is given by the expression Z where-8 isexpressed in meters, 1 is the frequency in'cycles per second, ,u. is thepermeability of the metal in henries per meter, and a is theconductivity in. mhos per. meter. The factor. 8 measures the distance inwhich the current or field penetrating into a slab of the metal manytimes 6 ,in thickness will decrease by one neper; i. e., their amplitudewill become .equal to 1/e=0.3679 times their amplitude at the surface ofthe slab. It is pointed .out in the parent application that when aconductor has .such a laminated structure, a wave propagating along theconductor at a velocity in the neighborhood of'a certain critical valuewill penetrate further into vthe conductor (or completely through it)than it would penetrate into a. solid conductor of the same material,resultingtin a more uniform current distribution in the laminated ,con-

ductor and consequently lower losses. The critical velocity'for'the typeof structure just described in determined by the thickness of the metaland insulating laminae and the dielectric constant of. the insulatinglaminae in the composite conductors. The critical velocity can bemaintained'by making the dielectric constant of the main-dielectric,that is, the dielectric material intermediate the two coaxial conductorsone or both of which may be "composite conductors, equal to -=where 6,is 'the dielectric constant of the-main dielectric element-between 'the'two conductors in farads per meter, e, is the dielectric constant of theinsulating material be- "tween the'laminae of the conductors in faradsper meter,

" w is the thickness of one of the metal laminae in meters,

"and tis the thickness of an insulating layerin meters.:Th'einsula'ting' layers are also made'very thin and an opti- -=-mumthickness for certain types of structures in accordance with theinvention is that in which each insulating layer'is one-half of thethickness of ametal lamina.

Forlong cables of the type just described, the main dielectric memberpreferably is of a dielectric of the proper'dielectric'constant to giveoptimum velocity of propagation: and which completely fills the spacebetween thertwo coaxially arranged conductors. However, it is :sometimesnot practical to maintain the rather accurate 'controlsnecessary toprovide the exact'dielectric constant required. The'present invention,inone of its more im- -portant aspects,-relates to a structure of thecomposite sconductor'type in which the intermediate or main dielec--:tric:member'does not take up all the space between the of \oneor more:hollow dielectric cylinders surrounding coaxial conductors. By .way ofexample,'it can be formed the inner composite conductor but taking onlypart of the -:.space':between the twoconductors, and a multiplicity ofdielectric'spacers associated with the dielectric cylinder or cylinders.By varying the number and/or positions rof the cylinders and spacers,the dielectric constant of the ferring'to'the following descriptiontaken in connection -:-with the accompanying drawings forming apartthereof,

in which:

Fig. 1 is an end View of a coaxial transmission line in accordance withthe invention, the inner conductor of the .ilinencomprising amultiplicitybfmetal laminations insu- :lated from one another and theinner and outer conductorsbeing separated by a composite dielectric.member;

.Fig. 2 .is a longitudinal cross-sectional view-of a section of cableofthe type shown in Fig. l;

Fig. 3 is a longitudinal cross-sectional view of a modification of theconductor of Figs. 1 and 2;

Fig. 4 is a graph of attenuation vs. dielectric constant of theinsulating medium between the inner and outer conductors of (A) acoaxial cable of the type shown in Fig. l and (B) a coaxial cable of aconventional type;

Fig. 5 illustrates a method of terminating coaxial cables of the typesshown in Figs. 2 and 3 where each of these cables has a metallic core;and

Fig. 6 shows a method of terminating the same types of coaxial cables inthe cases where the inner core is of dielectric material.

Referring more particularly to the drawings, Figs. 1 and 2 show, by wayof example, a conductor 10 in accordance with the invention, Fig. 1being an end view and Fig. 2 being a longitudinal view. The conductor 10comprises a central core 11 (which may be either of metal or dielectricmaterial but which by way of example has been shown as of metal), aninner composite conductor or stack 12 formed of many laminations ofconducting material 13 spaced by laminations of insulating material 14,and an outer conductor 15 separated from the inner conductor 12 by acomposite intermediate dielectric member 16. The dielectric member 16comprises an inner dielectric cylinder 17 and a plurality of dielectricspacers 18 between the members 17 and 15.

As discussed in the above-mentioned parent application, each of theconducting layers 13 is made thin compared to its appropriate skin depth(6). The insulating layers 14 are also preferably but not necessarily ofcomparable thinness with the conducting layers. Examples of satisfactorymaterials are: conductors-copper, silver and aluminum;insulators-polyethylene, polystyrene, quartz and polyfoam. The innerconductor 12 has 10 or 100 or more conducting layers 13 while the outerconductor 15 in the embodiment of Fig. 2 is a solid cylinder. Sincethere are a large number of insulating and conducting layers in thestack 12, it makes no diflierence whether the first or last layers ofthe stack is of conducting or of insulating material.

The improvement forming the present invention is based on the novelconstruction of the intermediate dielectric member 16. Since dielectricmembers 17 and 18 take up only part of the annular space between thestack 12 and the conductor 15, a material can be used which has a higherdielectric constant than that used in the composite conductor employinga main dielectric member completely filling the space between the twocoaxial conductors and still have an over-all dielectric constant forthe entire annular space which is of the proper value to produce avelocity of propagation which matches that in the stack and thus satisfythe so-called Clogston condition represented by Equation 2 given above.The cylinder 17 and the discs 18 can be of a material which has arelatively high dielectric constant and'still have an average dielectricconstant which can be'varied between values of one and over three, asshown in the curves in Fig. 4.

Fig. 4 compares the attenuation of a selected length of a line of thetype shown in Fig. 2 (curve A) with that of a conventional line (curveB) having a solid inner conductor of the same diameter, as thedielectric constant of the insulating material between the inner andouter conductors is varied by changing the number and spacing of thediscs 18. The attenuation of the conductor 10 is seen to reach a minimumat 2:; where 2 has the value given by the following equation 7 2:5 Thesimilarity between this equation and Equation 2 is obvious. This minimumvalue of attenuation at: is much less than that of the conventionalcoaxial conductor. Even for values of e appreciably difierent than I (asshown in Fig. 4), the conductor 10 has advantages over the conventionalcoaxial cable.

Fig. 3 is a longitudinal view of another embodiment of the invention. Inthis embodiment, a stack 21 replaces the outer solid cylinder 15 of thearrangement of Fig. 2. This stack can be similar to the stack 12 and bemade up of multiplicity of alternately arranged insulating andconducting laminae 22 and 23, respectively. The inner core 24 can beeither of metal or of dielectric material but, by way of example, it isbeing shown as a dielectric member. While one dielectric cylinder may beused surrounding the inner stack 12 (corresponding to the cylinder 17 ofFig. 2), the arrangement of Fig. 3 comprises two dielectric cylinders 25and 26 between which are placed the dielectric spacers 18. Obviously anynumber of dielectric cylinders in the intermediate space between thestacks 12 and 21 can be used. A shield 27 of any suitable metalsurrounds the outer stack 21. The members 25, 26 and 18 are chosen andspaced so that the over-all dielectric constant within the entire innerspace between the stacks 12 and 21 has the value represented by Equation3 above.

Figs. 5 and 6 show forms of terminations used with the conductorstructures shown in Figs. 2 and 3. In each case there is the problem ofconnecting one end of a composite conductor to another conductor in sucha way that there are the fewest possible impedance discontinuities ormismatches at the joint, although generally this condition is not acritical one. In certain cases, however, it may be necessary to providea very low loss cable of great length and with many intervening joints.It obviously would be advantageous to minimize mismatch and accompanyingmode conversion at the joints.

In the arrangement of Fig. 5, a cable such as cable 10 of Fig. 2 (or acable 20 of Fig. 3 with an inner core of metal) has its end positionedagainst an end of cable 30 of the coaxial type having an outer conductor31 and an inner conductor 32, the space between the conductors for ashort interval being filled with dielectric material 33. The outsidediameter of the cable 30 is chosen to be substantially the same as thatof the cable 10 so that the conductor 15 butts against and is connectedto the outer conductor 31 and the core 11 butts against and is connectedto the inner conductor 32. By means of this connection, a mismatchbetween the cable 10 and the cable 30 is prevented or kept at a minimum.The cable 30 can be connected at its other end to a source of energy orother utilization device 34.

Fig. 6 shows a connection between a conductor 20 of the type shown inFig. 3 (or a conductor 10 of Fig. 2 with an inner core of non-metallicmaterial) and a terminating cable 40. The outer conductor 41 buttsagainst and is connected to the outer sheath 27 of the cable 20 whilethe inner conductor 42 of the conductor 40 butts against the dielectriccore 24 of the cable 20. Dielectric material 43 is placed between theconductors 41 and 42 for a short distance. Since the dielectric core isnot of metallic material, the end of the inner conductor 42 flares outso that it contacts at least the inner one of the metal laminations 13of the stack 12. Alternatively, the inner conductor 42 can have adiameter large enough to contact at least the inner one of the metallaminations 13, in which case a flared end is not necessary. In bothcables 30 and 40, the dielectric members 33 and 43 are chosen so that ineach case it has approximately the same dielectric constant as that ofthe composite dielectric member in the composite conductor 10 or 20 towhich it is connected.

It is obvious that many changes can be made in the embodiments describedabove. The various embodiments and the modifications thereof describedherein are meant to be exemplary only and they do not by any meanscomprise a complete list of conductors to which the present invention isapplicable and it is obvious that others can be devised by one skilledin the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In combination, a medium for the transmission of high frequencyelectromagnetic waves comprising two coaxially arranged conductorsspaced apart, at least one of said conductors comprising a stack ofconcentric thinwalled conducting cylinders separated by layers ofinsulation, the space between said conductors having a plurality ofspaced dielectric members, the dielectric members having a dielectricconstant and being spaced in a manner such that the average dielectricconstant e of the space between the two conductors is givensubstantially by the following equation where 2 is the dielectricconstant of the insulating layers, t is the thickness of an insulatinglayer, and w is the thickness of one of said conducting cylinders.

2. In combination, a medium for the transmission of high frequencyelectromagnetic waves comprising two coaxially arranged conductorsspaced apart, at least one of said conductors comprising a stack ofconcentric thinwalled conducting cylinders separated by layers ofinsulation, the space between said conductors having a plurality ofspaced dielectric members, the dielectric members having a dielectricconstant and being spaced in a manner such that the average dielectricconstant e of the space between the two conductors is givensubstantially by the following equation where e is the dielectricconstant of the insulating layers, 2 is the thickness of an insulatinglayer, and w is the thickness of one of said conducting cylinders, andmeans for applying an electromagnetic wave to said transmission medium.

624,008 Germany Jan. 9, 1936

